Localized Surface Plasmon Resonance (LSPR)

Localized Surface Plasmon Resonance (LSPR) is a fascinating optical phenomenon that occurs when light interacts with metal nanoparticles, such as gold or silver, leading to the collective oscillation of electrons at the nanoparticle surface. This unique interaction results in strong light absorption and scattering at specific wavelengths, making LSPR highly sensitive to the size, shape, and surrounding environment of the nanoparticles.

Due to its remarkable properties, LSPR has become a powerful tool in various fields, including chemical and biological sensing, medical diagnostics, environmental monitoring, and surface-enhanced Raman scattering (SERS). Explore this page to learn more about the science behind LSPR, how it works, and its wide-ranging applications in cutting-edge technologies.

What is LSPR?

LSPR is a new generation of SPR technology based on gold nanoparticles. Unlike traditional SPR technology to detect the deflection of SPR Angle caused by the change of refractive index, LSPR detects the displacement of light absorption peak caused by the change of molecular layer thickness on the surface of gold nanoparticles. Because the change of light wavelength is little affected by the environment, it is not sensitive to the interference caused by changes in volume, temperature, and refractive index of the buffer.

Therefore, the detection results of this series of products not only ensure the accuracy and sensitivity similar to the traditional SPR technology, but also adopt a simpler optical path system and operation process, which greatly reduces the experimental threshold of SPR technology, and the renewable chip reduces the cost of later use. This series of advantages makes LSPR technology is expected to become a necessary technical platform support for life science research experiments.

Principle of LSPR

LSPR is a phenomenon that occurs when light interacts with metal nanoparticles, typically made of gold, silver, or other noble metals. This interaction leads to the collective oscillation of conduction electrons at the nanoparticle surface, resulting in a strong absorption and scattering of light at specific wavelengths. LSPR is highly sensitive to the size, shape, and dielectric environment of the nanoparticles, making it a powerful tool for various applications in sensing, biosensing, and environmental monitoring.

During LSPR, the free electrons in the metal nanoparticle collectively oscillate in resonance with the incident light, creating a localized enhancement of the electromagnetic field around the nanoparticle. This leads to two key optical effects:

• Strong absorption and scattering of light: The nanoparticle efficiently absorbs and scatters light at the resonant wavelength, which is typically in the visible or near-infrared region of the spectrum. This is why solutions containing metal nanoparticles often exhibit vibrant colors.
• Local field enhancement: The electromagnetic field around the nanoparticle is significantly amplified, which can enhance various optical processes, such as Raman scattering and fluorescence.

Key Features of LSPR

Localized resonance

Compared with traditional SPR, LSPR occurs on the surface of metal particles, and the oscillations of electrons are confined to the vicinity of nanoparticles.

Dependent on size and shape

The resonance wavelength of LSPR is closely related to the size and shape of metal particles, and the shape and size of nanoparticles (such as spherical, rod, triangle, etc.) will affect their resonance characteristics.

Sensitive to the environment

The resonant wavelength of the LSPR is also affected by the refractive index of the medium surrounding the particle, so it can be used to detect changes in the surrounding environment, such as gas concentration, liquid composition, or the binding of biomolecules.

What is LSPR Used For?

The LSPR wavelength and intensity are highly sensitive to the nanoparticle's properties, including its size, shape, material, and surrounding environment. This tunability makes LSPR a powerful tool for various applications, such as:

• Chemical and Biological Sensing: LSPR-based sensors can detect minute changes in the local environment, making them ideal for detecting chemical and biological molecules.
• Medical Diagnostics: LSPR is used in biosensors for the detection of biomarkers, pathogens, and other biological entities.
• Environmental Monitoring: LSPR sensors can be used to detect pollutants, toxins, and other environmental hazards.
• Surface-Enhanced Raman Scattering: LSPR enhances the Raman scattering signal, enabling the detection of molecules at very low concentrations.

How Does LSPR Work?

Localized Surface Plasmon Resonance arises from the coherent oscillation of conduction electrons in metallic nanostructures, driven by the electromagnetic field of incident light.   This phenomenon is fundamentally rooted in the interaction between light and matter at the nanoscale, where the dimensions of the metal nanoparticles are smaller than the wavelength of the incident light.

Specifically, LSPR works in the following steps:

Interaction Between Light and Metal Nanoparticles

When light interacts with metal nanoparticles, the electric field of the light induces a collective oscillation of the conduction electrons at the nanoparticle surface. This oscillation is known as a surface plasmon. For nanoparticles, these plasmons are localized, meaning they are confined to the surface of the nanoparticle rather than propagating along a metal-dielectric interface.

Localization of Surface Plasma

The localization of surface plasmons is a key feature of LSPR. Unlike propagating surface plasmons, which travel along the interface between a metal and dielectric, localized surface plasmons are confined to the nanoparticle surface. This confinement leads to a strong enhancement of the electric field near the nanoparticle surface.

Resonance Phenomenon

The resonance phenomenon in LSPR occurs when the frequency of the incident light matches the natural frequency of the electron oscillations in the nanoparticle. At this resonance condition, the absorption and scattering of light are maximized, leading to a distinct peak in the optical spectrum.

LSPR Wavelength Regulation

The LSPR wavelength can be regulated by changing the size, shape, and material of the nanoparticles, as well as the dielectric properties of the surrounding medium. For example, larger nanoparticles typically exhibit a red-shifted LSPR wavelength compared to smaller nanoparticles. Similarly, changing the shape of the nanoparticles from spherical to rod-shaped can also shift the LSPR wavelength.

The Influence of Environmental Media

The LSPR wavelength is highly sensitive to the refractive index of the surrounding medium. When the refractive index of the medium changes, the LSPR wavelength shifts accordingly. This sensitivity is exploited in LSPR-based sensors, where changes in the local environment (e.g., the binding of molecules to the nanoparticle surface) can be detected by monitoring shifts in the LSPR wavelength.

LSPR Compared with Traditional SPR

In contrast to the continuous gold film used in conventional SPRS, metal nanoparticles (usually gold or silver) generate local surface plasmon resonances. LSPR produces a strong resonant absorbance peak in the visible light range, the location of which is highly sensitive to the local refractive index around the particle. As a result, the LSPR measures small changes in wavelength at the absorbance position, rather than the Angle of light reflected from the gold film as a conventional SPR does. If you want to learn more about this, you can check out the article A Comprehensive Guide to Biacore Instruments Features, Specifications, and Applications.

Figure 1. LSPR and SPR basic schematic diagram. (Fig from Nicoya)

Both LSPR and SPR are optical phenomena based on surface plasmon resonance, but they differ significantly in principle and application.

LSPR mainly occurs on the surface of metal nanoparticles, and when light interacts with the nanoparticles, it excites localized electron oscillations, which are confined to the vicinity of the nanoparticles, producing strong local electric field enhancement. LSPR is very sensitive to the size, shape, and surrounding environment of nanoparticles and is suitable for localized high-sensitivity detection, such as single-molecule sensing and surface-enhanced Raman scattering.

In contrast, SPR occurs at the interface between a flat metal film and a medium and excites surface plasma waves to propagate along the interface. Sensitive to interfacial refractive index changes, SPR is suitable for studying a wide range of molecular interactions, such as protein binding and drug screening, but its electric field enhancement effect is weak and the experimental apparatus is often more complex.

LSPR is more suitable for localized, high-sensitivity detection scenarios, while SPR is more suitable for large-scale, high-precision molecular interaction studies. Both have advantages in areas such as biosensing, environmental monitoring, and chemical analysis, and the choice of technology depends on the specific application needs.

Detection and Application of LSPR

LSPR is widely used in various sensor technologies, medical diagnosis, and biomolecular detection due to its high sensitivity to free electrons on the surface of metal nanoparticles. The resonance wavelength of the LSPR changes with the refractive index of the surrounding medium, a property that makes it an extremely effective detection tool, especially for detecting small changes with high sensitivity.

Application of LSPR in Sensor

LSPR technology is widely used in various sensors, especially in chemical and biomolecular detection. By monitoring changes in the LSPR resonance wavelength, the presence and concentration of the target substance can be sensed. Common LSPR sensors include:

• Gas sensor: LSPR can be used for the detection of gas molecules. The interaction between gas molecules and the surface of metal nanoparticles changes the refractive index of the local environment, which causes the change of LSPR wavelength. This technology can be used to monitor the concentration of gases in the air, such as ammonia, carbon dioxide, etc.
• Liquid sensors: LSPR sensors can also effectively detect the chemical composition of liquids. When the liquid is in contact with the metal nanoparticles, the refractive index of the surface of the particle is changed, resulting in a shift in the wavelength of the LSPR, which is then used to analyze the chemical composition of the liquid.

Application of LSPR in Biosensor

The high sensitivity of LSPR makes its application in biosensors particularly outstanding. It can detect small changes in the binding of biomolecules to the surface of metal nanoparticles, such as the recognition and detection of DNA, proteins, antibodies/antigens, and other biomolecules. Common applications include:

Biomolecular detection: LSPR can be used to detect specific biomolecules such as DNA, RNA, proteins, antigens, and antibodies. By monitoring the change in LSPR wavelength, the concentration, interaction, and dynamic changes of these molecules can be analyzed. For example, testing for specific viruses or bacteria can help make accurate judgments in medical diagnosis.

Disease diagnosis: In medical diagnosis, LSPR is used as a portable and real-time diagnostic tool, capable of rapidly detecting pathogens and cancer markers, and more in body fluids such as blood and urine. LSPR sensors can detect early symptoms of cancer, heart disease, or other diseases, providing a non-invasive, low-cost way to detect them.

Application of LSPR in Environmental Monitoring

The sensitivity of LSPR technology also makes it an important application in the field of environmental monitoring. By using LSPR sensors for water quality, air quality and other monitoring, pollutants and harmful substances can be detected in real time and accurately.

Water quality monitoring: LSPR can detect harmful substances in water, such as heavy metal ions, pesticide residues, etc. When pollutants in the water interact with metal nanoparticles, the LSPR wavelength changes and the concentration of pollutants in the water can be quantitatively analyzed by this change.

Air quality monitoring: LSPR sensors can also be used to detect harmful gases or pollutants in the air, such as nitrogen dioxide, sulfur dioxide, and other gases, to help monitor air quality and prevent environmental pollution.

Application of LSPR Combined with SERS

LSPR is also used in combination with surface-enhanced Raman scattering technology to enable molecular detection at higher sensitivity. By enhancing the Raman scattering signal of molecules through the LSPR effect, trace molecules can be detected, thus improving the detection ability of traditional Raman scattering technology. This combination is widely used in:

Chemical detection: Enhanced Raman signal through LSPR can detect lower concentrations of chemical substances, which can be used in drug analysis, poison detection, and other fields.

Molecular recognition: In the biological field, the combination of LSPR and SERS can improve the recognition ability of biological molecules (such as viruses, bacteria, and proteins).